GPR vs EML — Which Detection Method Is Right for Your Utility Survey?
GPR (Ground Penetrating Radar) and EML (Electromagnetic Location) detect different types of buried services and work best in combination. GPR is better for non-metallic services — plastic pipes, ducts, and voids. EML is better for metallic services and live cables that can be traced directly. Used together as part of a PAS 128 Type B survey, they provide the most complete picture of what's below the ground.
GPR and total station equipment deployed on site — two of the key tools used in a comprehensive utility survey approach.
Two Methods, Two Different Jobs
One of the most common misconceptions about utility detection is that GPR and EML are alternatives — that you choose one or the other. In practice, they detect different things and complement each other directly.
Understanding what each method does and what it misses is important for anyone commissioning a utility survey, managing a project with excavation works, or specifying what PAS 128 investigation is required.
How GPR Works
Ground Penetrating Radar sends electromagnetic pulses into the ground and measures how they reflect back from interfaces between different materials — pipe walls, voids, changes in soil composition. The result is a continuous subsurface profile showing anomalies that may represent buried services, voids, or obstructions.
GPR is particularly effective for:
✔ Non-metallic services — plastic water pipes, HDPE ducts, clay drainage that EML cannot detect
✔ Voids and obstructions — collapsed drainage, unmapped culverts
✔ Dense service corridors where multiple pipes are present
✔ Providing approximate depth as well as horizontal position
GPR is less effective in saturated or clay-heavy ground where signal attenuation is high, and in areas with reinforced concrete which scatters the signal.
How EML Works
Electromagnetic Location works by detecting the electromagnetic field produced by a current flowing through a conductor. The current can either be the signal already present on a live cable, or it can be applied directly to a metallic pipe or cable using a signal generator.
EML is particularly effective for:
✔ Metallic services — gas mains, water mains, copper cables, steel pipes
✔ Live electrical and telecoms cables where the signal can be picked up passively
✔ Tracing individual services through congested areas using direct connection
✔ Confirming the route of a known metallic service
EML cannot detect non-metallic services — plastic pipes, clay drainage, concrete ducts — and is less useful where services are deep, poorly conductive, or shielded by other infrastructure.
Why You Need Both
A GPR-only survey will miss metallic services where signal clarity is poor, and an EML-only survey will miss all non-metallic services entirely. On a typical UK urban site with mixed service types — which describes most construction and highways projects — neither method alone provides adequate coverage.
This is precisely why PAS 128 Type B detection specifies both methods as the standard for a compliant utility investigation. The combination produces a significantly more complete picture than either can achieve alone, and it allows the PAS 128 confidence level assigned to each located service to be based on corroborating evidence from both methods.
The BSI (British Standards Institution) sets out in PAS 128 how these methods should be combined, recorded, and reported to produce defensible confidence levels rather than best-guess estimates.
When One Method Might Be Sufficient
There are specific circumstances where a single method may be appropriate — but these are the exception rather than the rule:
→ A known metallic service in a specific location being confirmed only — EML direct connection may be sufficient
→ A void detection exercise on a specific structure — GPR alone may be appropriate
→ A desktop records search only (PAS 128 Type D) where physical detection is not yet required
For any excavation project where the ground has not been physically detected, using both methods as part of a PAS 128 Type B investigation is the appropriate standard.
What This Means in Practice
When commissioning a utility survey, specifying 'GPR only' or 'EML only' leaves gaps that increase both safety and commercial risk. The right question to ask is not 'which method?' but 'what standard of investigation does this excavation require?' — and for most UK construction and highways projects, the answer is PAS 128 Type B using GPR and EML in combination.
Further background on how electromagnetic methods interact with ground conditions is published by the British Geological Survey.
Not sure what level of utility investigation your project needs?